Studies of platelet GPIb-alpha and von Willebrand factor bond formation under flow
Understanding the differential bonding mechanics underlying bleeding disorders is of crucial importance to human health. In this research insight is provided into how four of these bleeding disorders (each with somewhat similar clinical characteristics), work at the molecular bond level. The bleeding diseases studied here can result from defects in the platelet glycoprotein (GP) Ibα, the von Willebrand factor (vWF) molecule, or the ADAMTS-13 enzyme. Types 2B and 2M von Willebrand Disease (VWD) result in excess bleeding, yet type 2B has increased binding affinity between platelet GPIbα and vWF, while type 2M has decreased binding affinity between these two molecules. Platelet type VWD (pt-VWD) causes mutations in the GPIbα molecule and has similar characteristics to type 2B VWD. Further, in thrombotic thrombocytopenic purpura, bleeding results when there is a lack of active ADAMTS-13 enzyme. Each disease results in patient bleeding, but due to different mechanisms. This dissertation will explore the bonding mechanics between GPIbα and vWF and how they are altered in each disease state. To observe the GPIbα-vWF bonding mechanics, rolling velocities, transient tethering lifetimes, and tether frequency were determined using a parallel plate flow chamber. Data from these experiments suggest that wt-wt interactions are force dependent and have biphasic catch-slip bonding behavior. The data show that the shear stress at which the maximum mean stop time occurs differs between gain-of-function and loss-of-function mutations. Using similar methods, we study the changes resulting from pt-VWD mutations in GPIbα, and find that the catch bond seen for wt-wt interactions is lost for these mutations. Further, the data suggest that interactions with gain-of-function GPIbα mutations may be transport rather than force dependent. Finally, how the GPIbα-vWF tether bond changes for thrombotic thrombocytopenic purpura was also investigated to show that the bond lifetime in the absence of the enzyme is increased presenting a possible rationale for why bleeding occurs in this disease. Overall, the data show how the bonding mechanics of the GPIbα-vWF tether bond differ in four bleeding diseases. Further, these observations offer potential explanations for how these changes in the bonding mechanism may play a role in the observed patient bleeding.